This invention relates generally to high-voltage sources for gas ion generators, and more particularly to a source adapted to supply a unipolar voltage wave to a gas ionization electrode so as to generate copious amounts of ions without, however, producing deleterious chemical by-products.
Many industrial, therapeutic and research applications exist for gas ion generators, all of which include a discharge or ionization electrode to which a high voltage is applied. Though the present invention is useful in conjunction with all known forms of gas-ion generators, such as those used industrially for electrostatic separation and electrostatic coating or imaging, for purposes of explanation and analysis we shall consider the ionization problems encountered in an electrostatic precipitator for removing suspended particles from a gas by ionically charging the particles.
In an electrostatic precipitator, unipolar ions are produced by a discharge electrode, the ions migrating across the gap between this electrode and a collector electrode under the influence of an electric field established therebetween. In so migrating, the ions attach themselves to the aerosol particles moving with the gas passing between the electrodes, the charged particles being attracted to the collector.
In one elementary form of electrostatic precipitator in widespread use, the discharge electrode is a wire coaxially supported within a tubular collector electrode. This wire has a much smaller radius of curvature than the tubular collector, the air gap or inter-electrode space between these electrodes being very large compared to the radius of the wire. When, therefore, a voltage is impressed across these electrodes and the potential difference therebetween is raised, a point is reached where the air near the more sharply-curved discharge electrode breaks down, but only to an extent producing a corona discharge.
The electric field varies inversely with the radius of the wire. For a given air gap dimension, the level of voltage needed to produce a corona discharge is below that necessary to completely break down the dielectric of air to produce a spark discharge across the gap. Since an understanding of this distinction is vital to the invention, the behavior of corona and spark discharges will be further analyzed.
A corona discharge is a highly active glow region surrounding a discharge electrode. In the above described elementary form of precipitator, this electrode is constituted by a wire, the glow region extending a short distance beyond the wire. Assuming that the wire is negatively charged, the free electrons in the gas in the region of the intense electric field surrounding the wire gain energy from this field to produce positive ions and other electrons by collision. In turn, these new electrons are accelerated and produce further ionization.
This cumulative process results in an electron avalanche in which the positive ions are accelerated toward and bombard the negatively-charged wire. As a consequence of such ionic bombardment, secondary electrons are ejected from the wire surface which act to maintain the discharge. Moreover, high-frequency radiation originating from excited gas molecules lying within the corona region contribute to the supply of secondary electrons.
The electrons emitted from the negatively-charged wire or discharge electrode are drawn toward the positively-charged collector electrode. As these electrons advance into the weaker field away from the wire, they tend to form negative ions by attaching themselves to neutral oxygen molecules. These negative ions create a dense unipolar cloud that occupies most of the gap between the electrodes and constitutes the only current in the entire space outside the corona glow region. This space charge functions to retard the further emission of negative charge from the corona region and in this way restricts the ionizing field adjacent the wire, thereby stabilizing the discharge.
The type of corona produced depends on the polarity of the discharge or ionizing electrode. In the example given above, we have assumed a negative polarity, in which case positive ions are accelerated toward the electrode and negatively-charged oxygen ions are repelled therefrom to produce a corona discharge. Conversely, when the polarity of the ionizing electrode is positive, negative ions are accelerated toward the electrode, causing the breakdown of air molecules with the result that positive ions are repelled outward from the ionizing electrode to create a corona glow.
When, however, the voltage applied to the ionizing electrode is further elevated to a level exceeding the point at which a corona discharge is maintained in a stable condition, the air dielectric then completely breaks down, as a result of which the air in the gap is rendered relatively conductive to sustain a spark discharge which is accompanied by a heavy current flow.
An electrostatic precipitator attains its highest operating efficiency under optimum ionization conditions when the voltage applied to the discharge electrodes approaches the point of transition between an incomplete breakdown or corona discharge producing a copious supply of ions and complete air dielectric breakdown or spark discharge which effectively short circuits the precipitator and renders it inoperative.
But in practice one must be careful to apply a voltage to the ionizing or discharge electrode of a precipitator which is well below the level at which complete air breakdown is experienced, for the air breakdown characteristics of air in a precipitator varies with the nature and concentration of the pollutants therein as well as barometric pressure conditions. Moreover, the breakdown of the dielectric of air produces chemical reactions which constitute as serious health hazard; for this breakdown gives rise to toxic ozone and harmful oxides of nitrogen. But quite apart from this health hazard is the fact that ozone is highly reactive with electrical insulation and other structures and therefore has a destructive effect on the associated equipment.
In this explanation, we have assumed that the gas being ionized is free air. But the problems arising from the concomitant production of deleterious by-products is not limited to air, for the ionization of other gases is accompanied by undesirable chemical by-products when spark discharges are produced.